1. Field of the Invention
The present invention relates to a displacement detecting device which detects the amount of displacement (movement) of a diffraction grating (a scale) by using interference of light.
2. Description of the Related Art
Conventionally, displacement detecting devices having a scale and a detection head are widely used as measuring instruments for accurately measuring linear displacement, rotational displacement and the like. In recent years, displacement detecting devices using the light emitted from a light-emitting diode or laser are used. Further, there is a demand for high-resolution displacement detecting devices capable of measuring a displacement of 1 nm or less.
One of these conventional displacement detecting devices is described in, for example, Japanese Unexamined Patent Application Publication No. 2009-257841. In the displacement detecting device described in Japanese Unexamined Patent Application Publication No. 2009-257841, the light emitted from a light source is split into two beams by a beam splitter, the two beams of light are respectively reflected by two reflectors, and the reflected two beams of light are irradiated at the same position of a diffraction grating.
Further, the two beams of light irradiated on the diffraction grating are diffracted by the diffraction grating to generate two 1st-order diffracted lights. The 1st-order diffracted lights are each reflected by a reflector (such as a mirror) and irradiated on the diffraction grating again along an optical path identical to the optical path through which the light from the light source is irradiated on the diffraction grating. In such a manner, two 2nd-order diffracted lights are generated by being diffracted twice by the diffraction grating.
Next, the two 2nd-order diffracted lights are superimposed on each other by a beam splitter so as to interfere with each other, and the interference light of the two 2nd-order diffracted lights is imaged on a light receiving element. Further, an interference signal is detected by the light receiving element based on the interference light, and movement difference between the two 2nd-order diffracted lights is obtained based on the interference signal so as to detect the amount of the movement of the diffraction gratings.
Further, there is another displacement detecting device described in Japanese Unexamined Patent Application Publication No. 2007-218833, in which two beams of light obtained by splitting the light emitted from a light source by a beam splitter are irradiated at two different positions of a diffraction grating.